1. Technical Field
This invention generally relates to pulse width modulation (PWM) amplifiers and, more particularly, to incorporation of digital signal processing (DSP) into PWM audio power amplifiers.
2. Related Art
Analog pulse width modulation (PWM) for audio applications has been prevalent since the days of vacuum tubes. In recent years, the integration of digital signal processors (DSPs) or other digital modulators with a PWM stage for conversion from digital to analog has become increasingly popular. Typically, DSPs are used to directly control the pulse widths of the output power stage(s) as described by, for example, xe2x80x9cUltra Low Distortion Digital Power Amplification,xe2x80x9d Mark Sandler, Jason Goldberg, Roderick Hiorns, Robert Bowman, Michael Watson, Peter Ziman, 91st AES Convention Preprint 3115(Y-3), October 1991 and U.S. Pat. No. 5,548,526 to Craven.
Although the application of DSPs to digitally control the formation of PWM provides significant benefits, limitations are also present in known approaches. For example, the pulse width of the output pulses is quantized based on multiples of a controlling clock period. As such, resulting granularity or coarseness otherwise present in the output is typically reduced by noise-shaping methods relying on timers to statistically decrease the quantizing error, as described by, for example, xe2x80x9cToward the 24-bit DAC: Novel Noise-Shaping Topologies Incorporating Correction for the Nonlinearity in the PWM Output Stage,xe2x80x9d Peter Craven, Journal of the AES, Vol. 41 No.5, May 1993. Noise-shaping methods, however, may introduce noise into the output which greatly increases the out-of-band noise and somewhat increases the in-band noise.
In digital control of a PWM stage, the ability to effectively apply negative feedback to the power output stage is greatly reduced by latencies due to conversion of analog output signals to digital data and latencies in the subsequent numeric processing of the feedback signal. The ability to bound high-frequency output noise requires that the amount of high-frequency feedback be maximized. Any attempt to digitize a feedback signal results in added delays or phase lags in the fed back signals and adversely affect the bounding of the high-frequency output noise. To add to the bounding difficulty, the noise floor of A/D converters is often required to be at least 120 dB below full scale, else it becomes the limitation to the output noise performance.
Known approaches to implementing feedback with PWM output audio signals require that the PWM modulation spectra be largely removed from any signal that would attempt comparison to the input signal. If this is not done in these approaches, the use of feedback may actually increase the distortion levels over and above distortion levels found in open-loop PWM audio power amplifiers. As a consequence, digitally modulated PWM designs typically control with open-loop feed-forward corrections. Unfortunately, none of these approaches offer any hope of improving the random noise floor created by jitter in the gate drive and power circuits required to implement a feed-forward circuit.
A PWM output stage has total sensitivity to the voltage on its power supplies. For a feed-forward controlled PWM output stage this either requires precise regulation of the power supplies and/or feedback using fast A/D conversion of the power supply voltage to the digital modulator to minimize distortion. Although helpful in minimizing distortion, regulated power supplies are significantly more costly than non-regulated supplies. In addition, a low-latency A/D converter typically includes more analog circuitry than in a conventional analog PWM modulator. As such, the desire to remove analog circuitry through the use of the digital modulator actually increases the amount of analog circuitry in the system.
The output noise levels of any amplifier are at their lowest when there is little or no input signal. PWM amplifiers are no exception. Even with little or no input signal, however, PWM amplifiers may still exhibit output noise levels due to very small amounts of timing jitter in the PWM signal capable of producing large effective output signals. The higher the rail voltage (e.g. the bigger the amplifier) the more pronounced the jitter may become. The jitter noise in the output is the sum of both audio frequency jitter in the input signal as well as intermodulation terms from higher frequencies that may enter the PWM signal or modulation process.
Although prior art combinations of a DSP and a PWM output stages provides adequate performance and sufficiently low distortion for many non-critical applications, the above-described limitations become more significant in high fidelity applications. In such demanding applications, the noise performance of PWM power amplifiers is often marginal and additional noise is not tolerable. In addition, computations involved in digital PWM renderings detract from the ability to perform other computations with the DSP that may be useful in fidelity improvements. For demanding applications the noise and distortion imperfections of the power stage may require more exacting control than is possible with known approaches. Accordingly, there is a need for a PWM output stage for high fidelity applications capable of compensating for distortion by overcoming the previously described limitations.
This invention provides a pulse width modulated (PWM) audio power amplifier capable of minimizing distortion and optimizing fidelity in demanding high fidelity audio applications. The PWM audio power amplifier may include an interleaved power output stage cooperatively operating with a digital signal processor (DSP). Utilizing the combination of the signal processing capability of the DSP and the capability of the interleaved output stage to operate with a total switching duty of greater than unity, distortion and other noise may be minimized. In addition, substantially unregulated power supplies may be utilized without excessive distortion.
In one embodiment, the PWM audio power amplifier includes a power digital to analog converter (DAC) and a DSP. The DSP operates with feed forward control along a fully digital signal path to drive the power DAC based on an input signal. Digital control signals may be provided by the DSP along a fully digital signal path to an opposed current amplifier (OCA) included within the power DAC to convert uniformly sampled input signals to a continuous equivalent waveform. The digital control signals may be independently processed by the DSP to independently control the switching duty of output stage switches included in the OCA. Feed forward control of the switching duty may include consideration of the output voltages and currents of the converter as well as power supply control voltages to minimize distortion while optimizing efficiency.
In another embodiment, the PWM audio power amplifier includes a power DAC, a feedback control circuit and a DSP. The PWM audio power amplifier of this embodiment is a hybrid design taking advantage of the strengths in the digital domain as well as those in the analog domain. The power DAC in cooperative operation with the feedback control circuit performs analog signal processing that includes high frequency negative feedback control to minimize error in the output signal. The feedback control loop may include consideration of the voltage and current of the output signal as well power supply control voltage. In addition, the DSP 18 may perform digital signal processing of the input signal along with fully digital feed forward control to inject a feed forward error correction into the feedback control loop. As such, the entire VI output plane over the range of load impedance of the transducer 12 may be compensated for distortion, thereby providing a low-noise and low distortion output signal.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.
The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principals of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.
FIG. 1 is a block diagram of one embodiment of a pulse width modulated (PWM) audio power amplifier electrically coupled with a transducer.
FIG. 2 is an expanded block diagram of one embodiment of the PWM audio power amplifier illustrated in FIG. 1.
FIG. 3 is a block diagram of another embodiment of a PWM audio power amplifier electrically coupled with a transducer.
FIG. 4 is an expanded block diagram of one embodiment of the PWM audio power amplifier illustrated in FIG. 3.